Attention has been focused on belt-type fixing devices that can reduce thermal capacity in an image forming apparatus. Attention has also been given in recent years to induction heating (IH) type ones that can perform rapid heating or high-efficient heating.
In a fixing device that includes a heating rotary belt which generates heat by using electromagnetic induction heating, one proposed approach for suppressing an excessive temperature rise in a paper unpassing region (second region) outside a paper passing region (first region) in which paper as a recording medium passes when being conveyed (made to pass) through the fixing nip is by controlling the amount of heat generation of a heating rotator in the paper passing region and that in the paper unpassing region in response to the length of the paper (paper width) in a direction substantially perpendicular to the paper conveyance direction.
A fixing device utilizing electromagnetic induction heating and that includes a heating roller (heating rotator) being a metallic roller that includes a magnetic shunt alloy layer and an induction coil arranged inside the heating roller has been proposed.
For the proposed fixing device, in a paper unpassing region having an increased temperature resulting from paper not passing through that region, when the magnetic shunt alloy layer of the metallic roller is at or above the Curie temperature, the magnetic properties of the magnetic shunt alloy layer can be lost. The loss of the magnetic properties of the magnetic shunt alloy layer enables suppressing an excessive temperature rise in the paper unpassing region of the heating roller in response to the width of passing paper of each size.
However, for the proposed fixing device, a movable magnetic-flux blocking member for controlling the amount of heat generation of the heating rotary belt is arranged outside the heating rotary belt. This may lead to a large size of the fixing device.
For the proposed fixing device, it is necessary to make the magnetic shunt alloy layer of the heating roller thicker than a predetermined value. This may lead to increased thermal capacity of the fixing device.
Accordingly, it is desired that the fixing device be able to control the amount of heat generation of the heating rotator in the paper unpassing region and that in the paper passing region in response to each of the sizes of paper, and also be able to reduce (or at least avoid increasing) thermal capacity and suppress an increase in the size.